This simulation demonstrates the role that friction plays in driving a car. In this case, the car is a front-wheel-drive car. With no friction at all, the engine spins the front wheels but the front wheels just slip on the frictionless road and the car does not move. If there is friction acting on the front wheels only, the force of static friction acts to the right to prevent the front wheels from slipping - this is the force that propels the car forward. Friction on the rear wheels acts left, and is much smaller than friction acting on the front wheels. For the rear wheels, static friction just needs to provide the torque needed to keep the rear wheels rolling without slipping.